In the glass industry, for example, there are applications where a transparent material such as glass must be measured for thickness for the manufacturing of windows or for the location of coated surfaces such as the surface location of a LOW-E energy efficient coating that has been applied to one side of the glass. One method for measuring glass properties is to direct a light beam at an angle to a surface of the glass and sensing the location and intensity of reflections of the light beam from the surfaces of the glass. The reflections will be influenced by factors including the spacing between the glass surfaces, the presence, location and nature of any coatings on the glass surfaces and the wave length of the light beam.
The light beam and the reflected light signals will often be transmitted through a path which includes contaminated air, water, and the coated transparent surfaces being tested. The signal quality received from an optical system working in multiple environments that may be affected by air borne or liquid contaminants must be preserved to reduce the amount of labor necessary to maintain that system. Different environments in which glass properties are measured may include, for example, glass cutting tables with the possibility of air borne glass particles, large factory environments with HVAC or fan systems moving contaminated air through the environment, liquids that are used to wash the glass to remove Lucor (separating powders) and oil residues from the glass. Routine factory maintenance is used to clean the manufacturing area, but this can leave the possibility of lower quality product being manufactured as the sensing environment is diminished until a failure condition occurs.